A thin film transistor (TFT) is a three-terminal element having a gate terminal, a source terminal, and a drain terminal. It is an active element in which a semiconductor thin film deposited on a substrate is used as a channel layer for transportation of electrons or holes and a voltage is applied to the gate terminal to control the current flowing in the channel layer and switch the current between the source terminal and the drain terminal. Currently, the most widely used TFTs are metal-insulator-semiconductor field effect transistors (MIS-FETs) in which the channel layer is composed of a polysilicon or amorphous silicon film.
Recently, development of TFTs in which ZnO-based transparent conductive oxide polycrystalline thin films are used as the channel layers has been actively pursued (Patent Document 1). These thin films can be formed at low temperatures and is transparent in visible light; thus, flexible, transparent TFTs can be formed on substrates such as plastic boards and films.
However, known ZnO rarely forms a stable amorphous phase at room temperature and mostly exhibits polycrystalline phase; therefore, the electron mobility cannot be increased because of the diffusion at the interfaces of polycrystalline grains. Moreover, ZnO tends to contain oxygen defects and a large number of carrier electrons, and it is thus difficult to decrease the electrical conductivity. Therefore, it has been difficult to increase the on/off ratio of the transistors.
Patent Document 2 discloses an amorphous oxide represented by ZnxMyInzO(x+3y/2+3z/2) (wherein M is at least one element selected from Al and Ga, the ratio x/y is in the range of 0.2 to 12, and the ratio z/y is in the range of 0.4 to 1.4). However, the electron carrier concentration of the amorphous oxide film obtained herein is 1018/cm3 or more. Although this is sufficient for regular transparent electrodes, the film cannot be easily applied to a channel layer of a TFT. This is because it has been found that a TFT having a channel layer composed of this amorphous oxide film does not exhibit a sufficient on/off ratio and is thus unsuitable for TFT of a normally off type.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-298062    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2000-044236